This invention relates in general to the field of photolithography. In particular, the invention relates to photomasks used in photolithography and to methods for manufacturing and using such photomasks.
A photolithography system typically uses a photomask, also known as a xe2x80x9cmaskxe2x80x9d or xe2x80x9creticle,xe2x80x9d to transfer a pattern into a substrate. For example, manufacturers of integrated circuits (ICs) use photomasks as masters to optically transfer precision images from the photomasks onto semiconductor wafers. A photomask is a crucial component of a photolithography system because the photomask serves as the template that creates the image of a complex pattern, such as an integrated circuit layer, on a wafer.
To create a photomask, a photomask manufacturer may use a standard photolithography process, such as laser or electron beam lithography, to form the desired pattern on one surface of a high-purity quartz or glass plate, sometimes referred to as a xe2x80x9cphotomask blankxe2x80x9d or xe2x80x9csubstratexe2x80x9d. The photomask pattern is defined by areas that are covered by a chrome-based or other optical absorber and areas that are free of optical absorber. The former areas are referred to as chrome, dark, or opaque, while the latter are referred to as clear or glass. The pattern, sometimes referred to as the xe2x80x9cgeometryxe2x80x9d or xe2x80x9cimage,xe2x80x9d may include millions of individual, microscopic features.
One particular type of photomask is known as an Alternating Aperture Phase Shifting (AAPS) photomask. The manufacture of AAPS masks includes etching alternating areas of transparent substrate (areas which are free of optical absorber) in order to form so-called xe2x80x9ctrenchesxe2x80x9d in the substrate. The trenches are preferably designed to cause a phase shift in the electromagnetic radiation (EMR) that passes through the photomask. Such a phase shift advantageously results in sharp edge definition and consequent resolution improvement.
One problem associated with AAPS masks results from diffraction effects. In general terms, as light passes through the trench at a non-normal angle, some of the light will be refracted outside of the trenched area. These diffraction effects cause less light to exit the etched trenches than that exiting the unetched area, resulting in an unwanted intensity imbalance. Such an imbalance decreases the effectiveness of the photomask and detracts from the improvements which motivate the use of AAPS photomasks.
Therefore, as recognized by the present invention, a need therefore exists for a way to decrease the intensity imbalance caused by diffraction effects associated with AAPS photomasks. In accordance with the teachings of the present invention, disadvantages and problems associated with diffraction effects in AAPS photomasks have been substantially reduced or eliminated.
In a particular embodiment, a method for fabricating an AAPS photomask with improved intensity balance is disclosed that includes the operation of forming an alternating aperture phase shifting photomask pattern on a substrate, including forming trenches within the substrate. The method further includes forming a layer of antireflective material within the bottom of at least one trench. More particularly, the method includes forming the layer of antireflective material from Magnesium Fluoride (MgF2) or another material with a refractive index less than the refractive index of the substrate. Furthermore the antireflective layer may be formed using a vacuum evaporation technique. The layer of antireflective material formed at the bottom of the trench areas increases the transmission of light through the trench areas by improving light coupling into the trench. Another embodiment of the invention may include an enhanced AAPS photomask fabricated according to the above method.
A method for using a damage resistant photomask according to the present invention may be employed by a manufacturer of products such as integrated circuits. Such a method uses an enhanced AAPS photomask that features a pattern of opaque and clear areas and trenched areas layered with antireflective material. For instance, the manufacturer may project electromagnetic radiation through the clear areas and the protective layer of the photomask onto a wafer that has been coated with photoresist. The manufacturer may then develop the photoresist to leave a pattern of photoresist on the wafer that corresponds to the pattern of opaque and clear areas on the photomask.
The present invention included a number of important technical advantages. One important technical advantage; is forming a layer of antireflective material in the trenched areas of the AAPS photomask. The antireflective material decreases diffraction effects and aids in balancing transmission through etched and unetched areas of the AAPS photomask. Further advantages are described in the Claims, Figures, and Description below.